1. Field of the Invention
The present invention relates, generally, to implantable cardioverter defibrillators (ICDs) and defibrillation methods, and particularly to a method and apparatus for determining the optimal shock strength for defibrillation utilizing the upper limit of vulnerability (ULV). More particularly, the invention relates to methods of timing T-wave shocks for purposes of determining cardiac shock strength based on multiple signals within a single electrogram, plural electrograms, or a combination thereof. Most particularly, the invention relates to timing by processing plural signals utilizing qualitative signal measurement techniques and signal alignment techniques.
2. Background Information
Heart disease is a leading cause of death in the United States. The most common form of cardiac death is sudden, caused by cardiac rhythm disturbances (arrhythmias) in the form of a ventricular tachycardia or ventricular fibrillation.
Ventricular tachycardia is an organized arrhythmia originating in the ventricles. It results in cardiac contractions that are too fast or too weak to pump blood effectively. Ventricular fibrillation is a chaotic rhythm disturbance originating in the ventricles that causes uncoordinated cardiac contractions that are incapable of pumping any blood. In both ventricular tachycardia and ventricular fibrillation, the victim will most likely die of “sudden cardiac death” if the normal cardiac rhythm is not reestablished within a few minutes.
Implantable cardioverter defibrillators (ICDs) were developed to prevent sudden cardiac death in high risk patients. In general, an ICD system consists of implanted electrodes and a pulse generator that houses implanted electrical components. The ICD uses implanted electrodes to sense cardiac electrical signals, determine the cardiac rhythm from these sensed signals, and deliver an electrical shock to the heart if life-threatening ventricular tachycardia or ventricular fibrillation is present. This shock must be of sufficient strength to defibrillate the heart by simultaneously depolarizing all or nearly all heart tissue. Shock strength is typically measured as shock energy in Joules (J). The defibrillating shock interrupts the abnormal electrical circuits of ventricular tachycardia or ventricular fibrillation, thereby permitting the patient's underlying normal rhythm to be reestablished. ICD pulse generators are implanted within the patient and connected to the heart through electrodes to provide continuous monitoring and immediate shocking when a life-threatening rhythm disturbance is detected. Because the devices must be small enough for convenient implantation, ICDs are limited in their ability to store electrical energy. In general, ventricular tachycardia can be terminated by weaker shocks than those required to terminate ventricular fibrillation. Thus ICDs must deliver a sufficiently strong shock to insure reliable defibrillation in response to each occurrence of ventricular fibrillation.
It is well known in the art that the shock strength required to defibrillate a human heart effectively varies with the implanted lead configuration and placement as well as the individual heart's responsiveness to the shock. To maximize efficiency of an ICD system, the minimum shock strength necessary to defibrillate an individual patient's heart reliably must be determined. However, it is also well known in the art that the relationship between an ICD's defibrillation shock strength and success or failure of defibrillation is represented by a probability-of-success curve rather than an all-or-none defibrillation threshold (DFT). Very weak, low strength (low energy) shocks never defibrillate. Very strong shocks, at energies greater than the maximum output of ICDs, always defibrillate. However, clinically relevant shock strengths for ICDs lie between these two extremes. In this intermediate range of shock strengths, a shock of a given strength may defibrillate successfully on one attempt and not on another attempt.
Determining a complete curve of the probability of success for every possible shock strength requires many fibrillation-defibrillation episodes. In clinical (human) studies and procedures, the number of fibrillation-defibrillation episodes should be limited because of their associated risks. Thus the goal of testing at the time of ICD implant cannot be to determine a complete probability of success curve. In general, the goal of testing at ICD implant is to provide an accurate estimate of the minimum shock strength that defibrillates with a high probability of success while using a minimum amount of testing. The shock energy that defibrillates with an X % probability of success is referred to as the defibrillation thresholdX or DFTX. Thus a goal of clinical testing at ICD implantation is to estimate a shock strength in the range of the DFT95-DFT99. This is the optimal strength at which to program the first shock of an ICD. For research purposes, it may be preferable to estimate the DFT50.
The minimum measured shock strength that defibrillates during a given session of defibrillation testing is referred to, in general, by the term DFT, despite the fact that no true threshold for defibrillation exists. All methods for determining the DFT of an ICD system require inducing fibrillation a number of times and testing various shock strengths for defibrillation through the implanted defibrillation leads. In the commonly used step-down method defibrillation is attempted at a high shock strength that is likely to defibrillate the heart successfully. If this shock is unsuccessful, a stronger “rescue shock” is delivered to effect defibrillation. Regardless of the outcome of the defibrillation shock, there is a waiting period of about 5 minutes to permit the patient's heart to recover. If the defibrillation shock is successful, fibrillation is reinitiated and the defibrillation is attempted at a lower shock strength. This process is repeated with successively lower defibrillation shock energies until the shock does not defibrillate the heart. The minimum shock strength that defibrillates is the DFT. Depending on the initial shock strength, the DFT determined in this manner is usually between the DFT30 and DFT70. The ICD is then programmed to a first-shock strength selected to be an estimate of the lowest value that can reliably achieve defibrillation by adding an empirically-determined safety margin to the DFT.
Other methods for determining the DFT require additional fibrillation-defibrillation episodes after a defibrillation shock has failed. In these methods, fibrillation is reinitiated after a failed defibrillation shock and defibrillation is attempted at successively higher shock strengths until a shock defibrillates the heart successfully. This change from a shock strength that does not defibrillate to one that does (or vice versa) is called a reversal of response. DFT methods may require a fixed number of reversals. If the size of the shock increments and decrements is the same, a multiple-reversal (up-down) method provides a good estimate of the DFT50. An alternative Bayesian method uses a predetermined number of unequal shock increment steps and decrement steps to estimate an arbitrary, specific point on the DFT probability of success curve.
One significant disadvantage of all DFT methods is the necessity to repeatedly fibrillate and then defibrillate the patient's heart to determine the DFT. A second disadvantage is that successful defibrillation is a probability function of shock energy, not an all or none phenomenon described by a simple threshold.
It is known in the art that shocks delivered during the vulnerable period of the normal cardiac cycle induce ventricular fibrillation, providing that the shock energy is greater than a minimum value and less than a maximum value. The ULV is the shock strength at or above which fibrillation is not induced when a shock is delivered during the vulnerable period of the normal cardiac cycle. The ULV may be displayed graphically as the peak of the vulnerable zone, a bounded region in a two-dimensional space defined by coupling interval (time) on the abscissa and shock strength on the ordinate. The ULV, which can be measured in regular rhythm, corresponds to a shock strength that defibrillates with a high probability of success and correlates strongly with the DFT. Because the ULV can be determined with a single fibrillation-defibrillation episode, it has the potential to provide a patient-specific measure of defibrillation efficacy that requires fewer fibrillation-defibrillation episodes than DFT testing.
Although the vulnerable period occurs generally during the T-wave of the surface electrocardiogram (ECG), its precise timing varies from individual to individual. More importantly, the peak of the vulnerable zone, which corresponds to the most vulnerable time intervals in the cardiac cycle, also varies from individual to individual. Accurate determination of the ULV depends critically delivering a T-wave shock at the peak of the vulnerable zone.
Several methods of determining the defibrillation shock strength for ICDs are based on the ULV. U.S. Pat. No. 6,675,042 ('042 patent) to Swerdlow and Shivkumar, entitled Defibrillation Shock Strength Determination Technology, is incorporated by reference herein.
In one embodiment of the '042 patent, the ULV is determined by shocking the heart at a series of predetermined times in relation to the first temporal derivative of the T-wave and at increasing or decreasing test-shock strengths. The lowest shock strength which fails to induce fibrillation is determined to be the ULV. The optimal first shock strength for programming an ICD is predicted to be incrementally greater than the ULV by about 5 J. In a second embodiment, a vulnerability safety margin method, the heart is shocked at a series of predetermined times in relation to the first temporal derivative of the T-wave, but only at a single test shock energy. If fibrillation is not induced, a safe shock strength is predicted to be incrementally greater than tested shock strength by about 5 J. This safety-margin approach does not determine the minimum (optimal) safe shock strength, but rather only ensures that the programmed shock strength is sufficient. The advantages of the safety margin method are that a sufficient first shock strength for ICDs can be determined without inducing fibrillation and that only three to four test shocks are required. Research has shown that programming first ICD shocks to 5 J above the shock strength tested in this vulnerability safety-margin strategy resulted in a first-shock success rate as good as or better than those reported for other methods of implant testing. Research has also shown that this strategy, which does not require induction of fibrillation, can be applied to at least 80% of ICD recipients.
In another embodiment of the '042 patent, the intra-cardiac electrogram used for determining the derivative of the T-wave is recorded between one (or more) large intra-cardiac electrode(s) such as defibrillation coils and one (or more) extra-cardiac electrodes such as the ICD housing (commonly referred to as a “case” or “can”) or the ICD housing coupled to another defibrillation electrode such as a defibrillation coil in the superior vena cava.
One method of the '042 patent is for determining a therapeutic cardiac shock strength for defibrillation, and comprises the steps of:                (a) sensing a change with respect to time in the T-wave of a cardiac signal;        (b) delivering a test shock by:                    (i) delivering a test shock at a test-shock strength and at a test-shock time relating to the change with respect to time of the T-wave; and            (ii) sensing for cardiac fibrillation; and                        (c) if fibrillation is not sensed, repeating step (b) at the test-shock strength and at a different test-shock time relating to the change with respect to time of the T-wave; and        (d) if fibrillation is sensed, setting the therapeutic cardiac shock strength as a function of the test-shock strength.        
A particular method for determining an optimal programmed first-shock strength of an ICD relative to the ULV, the ICD having at least one sensing electrode and at least one shocking electrode, comprises the steps of:                (a) setting an initial test-shock strength, four offset times, and a shock strength decrement;        (b) delivering a set of up to four test shocks with the ICD to the patient, each test shock member of the set of test shocks comprising the sub-steps of:                    (i) sensing an electrogram from the patient;            (ii) detecting at least one predetermined base timing point prior to the T-wave of the electrogram;            (iii) differentiating the electrogram;            (iv) detecting at least one maximum of the derivative of a T-wave;            (v) measuring at least one base time interval from the at least one base timing point to the at least one maximum derivative of a T-wave;            (vi) delivering a test shock to the patient at the test-shock strength and at a test-shock time corresponding to the base time interval plus one of the offset times;            (vii) sensing for an induction of fibrillation for a predetermined sensing time period; and            (viii) if fibrillation is not sensed in step b (vii), then repeating sub-steps b (i-vii), at the same test-shock strength, up to the fourth test shock, each test shock member of the set of test shocks having a different test-shock time corresponding to a base time interval plus an offset time; and                        (c) if fibrillation is not sensed in step (b) by the fourth test shock, then repeating step (b) at a lower test-shock strength corresponding to the shock strength decrement, to deliver at least one additional set of up to four test shocks; and        (d) if fibrillation is sensed in step (b), then:                    (i) defibrillating the patient; and            (ii) setting the programmed first-shock strength of an ICD at a predetermined higher level than the test-shock strength at which fibrillation was induced.                        
One embodiment of the apparatus of the '042 patent is an overall ICD system which delivers an optimal therapeutic cardiac shock, comprising:                (a) a plurality of electrodes, at least one electrode being adapted for sensing cardiac signals and at least one electrode being adapted for delivering shocks to the heart;        (b) a shock subsystem connected to the at least one electrode for delivering shocks and which is capable of generating test shocks and therapeutic cardiac shocks; and        (c) a ULV subsystem connected to the shock subsystem and for providing test shock information to the shock subsystem, the test shock information including test-shock strength and test-shock time relating to a change in cardiac signals with respect to time, and for determining the shock strength of the therapeutic cardiac shocks as a function of the test shock strength.        
Another apparatus embodiment of the '042 patent is a ULV subsystem for determining a therapeutic cardiac shock strength, for example with an existing ICD, comprising:                (a) a sensor for sensing the electrical activity of the heart, including a change with respect to time of the T-wave of a cardiac signal and including the presence of fibrillation;        (b) a test-shock driver for transmitting time and strength information regarding test-shocks;        (c) a controller to determine the cardiac shock strength as a function of the response (fibrillation or no fibrillation) to test-shocks of varying strengths and times.        
A particular aspect of the apparatus of the '042 patent is an ICD system for determining and providing an optimal programmed first-shock strength based on the upper limit of vulnerability, comprising:                (a) a plurality of implantable electrodes; and        (b) a shock delivery subsystem for generating and delivering shocks, connected to the electrodes; and        (c) a ULV subsystem comprising;                    i) a sensor, connected to the electrodes, for sensing the electrical activity of the heart, including a change with respect to time of the T-wave of a cardiac signal and including the presence of fibrillation;            ii) a timer connected to the sensor for providing a series of shock times, timed relative to the maximum derivative of the T-wave;            iii) a test shock driver, connected to the timer, for transmitting timing and amplitude information regarding T-wave, test shocks;            iv) a memory unit, connected to the test shock driver and the shock subsystem, for storing programmable values such as pacing cycle length, timing intervals, an initial shock strength, and values for incrementing and decrementing shock strength; and            iv) a controller, connected to the sensor, test-shock driver, and shock subsystem for incrementally varying shock strength and the shock times, whereby the system provides a test shock having a shock strength and shock time selected by the controller, the shock subsystem delivering an initial test shock to the heart at an initial shock strength and an initial shock time and delivering subsequent test shocks to the heart by varying the shock and decreasing the shock strength, a strength decrement, until the heart is induced to fibrillate, whereby the shock strength of the test shock immediately prior to the test shock that induces fibrillation represents the ULV.                        